New breast cancer drug is ‘very exciting’

September 5, 2017

Researchers are encouraged by the early results for patients using Z-endoxifen. The potent derivative of the drug tamoxifen was given to women with estrogen-receptor-positive metastatic breast cancer, the most common form of breast cancer in women whose disease has spread. The phase I study proved endoxifen is safe and it shrinks tumors – even for patients whose cancer had continued to progress with standard estrogen therapies, including tamoxifen.

"As a proof of concept, obviously, this is very exciting," says Dr. Matthew Goetz, a Mayo Clinic oncologist and principal investigator on the study. "While the primary goal of the study was to safely deliver therapeutic levels of endoxifen, one of the most surprising observations was the prolonged anti-cancer benefit, [which] in some cases lasted more than two years in women whose cancer had progressed on standard hormonal-based therapies.”

The Mayo Clinic study was the result of a close collaboration with the National Cancer Institute. Mayo scientists worked to assist with the preclinical drug development to ultimately conduct the first-in-human trial of Z-endoxifen in women with metastatic breast cancer. “We can clearly say that endoxifen, as a drug, would have never happened without this partnership with the National Cancer Institute."

The findings of the clinical trial are published in the Journal of Clinical Oncology. Dr. Goetz says the next phase of research is already underway, directly comparing the effectiveness of Z-endoxifen with tamoxifen.